Field of the Invention
The invention pertains to a process for catalytically purifying the exhaust gas of a combustion plant fuelled with solid, liquid or gaseous fuel. A liquid or gaseous reducing agent, such as aqueous urea solution or gaseous ammonia, is injected into the hot exhaust gas stream in the process. The injection is effected with an atomizing medium, such as compressed air. The reducing agent is thereupon decomposed and a catalytic reduction (in particular by the SCR method) with the hazardous gaseous exhaust gas components is induced.
The invention is used in the catalytic cleaning of exhaust gas, in particular by the SCR process, of engines and fired plants, e.g. diesel engines, diesel-gas engines, gas engines in the lean mode, boilers fired with oil, gas, wood and other fuels, and gas turbines which are provided, for example, for driving generators, compressors, commercial vehicles, machines, ships and locomotives or, for example, for generating heat, steam and hot water.
U.S. Pat. No. 5,431,893 to Hug et al. (EP-B1 0 558 452), in particular FIG. 1 thereof, discloses a process and a device of above-mentioned type for cleaning exhaust gases from combustion plants. The prior art device comprises a housing having three mutually parallel ducts or chambers. A feed line for the hot exhaust gases to be cleaned opens laterally into the first end of the first duct, which is termed the pyrolysis duct. A two-component atomizing nozzle is disposed at the first end, roughly centrally. To this is fed a reactant, such as aqueous urea solution, and an atomizing medium, such as compressed air at a gauge pressure of 1 bar. The urea solution and the compressed air form an aerosol-like spray cone which is carried in the longitudinal direction of the pyrolysis duct. The urea is completely converted or broken down into finely divided ammonia and carbon dioxide. The exhaust gas stream containing the ammonia and the carbon dioxide passes, at the second end of the pyrolysis duct, into the second parallel duct, which is termed the mixing duct. The exhaust gas stream runs there in the opposite direction, more precisely through some conventional cross-flow mixers. The exhaust gas stream, which is at that point homogeneously and intensively mixed with the broken-down reactant is subsequently passed into the third parallel duct, which is termed the reaction duct. There it is first conducted through two reduction catalysts which are spaced apart a given distance from one another, preferably SCR catalysts (SCR=selective catalytic reduction). Then it is optionally conducted through an oxidation catalyst, likewise arranged at a distance thereto, and constructed with the same geometry. The reduction catalysts and the optional oxidation catalyst are of a honeycomb structure, that is equipped with longitudinal ducts. The exhaust gas stream then freed from all gaseous pollutants then passes into a heat exchanger or exits via an outlet, e.g. a stack or exhaust.
That prior art device for cleaning exhaust gases can be constructed in a highly compact system because of the parallel arrangement of the three ducts. However, for many applications, a still more compact construction with the same performance is desirable. This requirement is due to the fact that available space is often very limited, such as, for instance, in the case of motor vehicles.
German patent application DE-A1 42 03 807 discloses an exhaust gas cleaning system with a centrally symmetrical structure. However, no deflection is effected in that system. Due to the relatively excessive length of that system, it cannot be used in all motor vehicle types, for example. A more compact structure would be desirable.
It is accordingly an object of the invention to provide a process for catalytically purifying exhaust gas from a combustion plant, which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which allow for still more compact structures without sacrificing in terms of system performance.
With the foregoing and other objects in view there is provided, in accordance with the invention, a process for cleaning exhaust gas from a combustion plant fuelled with solid, liquid, or gaseous fuel. The process comprises the following steps:
a) introducing exhaust gas into a combined conversion and mixing duct and forming an exhaust gas stream through the duct along a predetermined longitudinal direction;
b) injecting reducing agent into the exhaust gas stream flowing through the conversion and mixing duct, and mixing the reducing agent with the exhaust gas and simultaneously converting the reducing agent;
c) providing a reaction duct parallel to the conversion and mixing duct, and deflecting the exhaust gas stream from the conversion and mixing duct into the reaction duct and causing the exhaust gas to flow in a direction opposite the predetermined longitudinal direction;
d) contacting the exhaust gas with a reduction catalyst in the reaction duct and reducing reducible exhaust gas components of the exhaust gas; and
e) discharging a stream of cleaned exhaust gas from the reaction duct.
The invention is based on the finding confirmed by testing that the first and second duct can, under certain boundary conditions, be functionally united, i.e., combined, to form a single duct, which is the combined conversion and a mixing duct. The combined duct may extend parallel to the reaction duct or inside the reaction duct (preferably centrally). A spatial separation into three ducts is therefore, surprisingly, not necessary for the proper function of the system.
In accordance with an added mode of the invention, the introducing step comprises forming a vortex flow in the exhaust gas stream upon introducing the exhaust gas into the combined conversion and mixing duct.
In accordance with an additional feature of the invention, the injecting step comprises injecting a reducing agent selected from the group consisting of liquid (e.g. aqueous urea solution) and gaseous (e.g. ammonia) reducing agents into the exhaust gas.
In accordance with another feature of the invention, the process further comprises a step of contacting the exhaust gas flowing in the reaction duct with an oxidation catalyst and/or contacting the exhaust gas with a hydrolysis catalyst.
With the above and other objects in view there is provided, in accordance with the invention, a device for cleaning exhaust gas originating from the combustion of solid, liquid, or aqueous fuel, comprising:
a housing having a combined conversion and mixing duct defined therein and being formed with an exhaust gas inlet for feeding exhaust gas into the conversion and mixing duct, the conversion and mixing duct defining a longitudinal direction along which the exhaust gas flows through the conversion and mixing duct;
a feed apparatus for injecting a reducing agent into the conversion and mixing duct;
the housing having a reaction duct formed therein extending substantially parallel to the conversion and mixing duct, and including a reducing catalyst disposed in the reaction duct;
a deflector disposed between the conversion and mixing duct and the reaction duct, the deflector deflecting the exhaust gas from the longitudinal direction in the conversion and mixing duct into an opposite direction in the reaction duct; and
the housing being formed with an outlet for outflowing cleaned exhaust gas from the reaction duct.
In accordance with again an added feature of the invention, the reaction duct surrounds the conversion and mixing duct externally.
In accordance with again another feature of the invention, the feed apparatus is a two-component atomizing nozzle for aqueous urea and compressed air disposed centrally at the inlet of the conversion and mixing duct.
In accordance with again an additional feature of the invention, there is provided a further deflector disposed between the reaction duct and the outlet.
In accordance with again a further feature of the invention, there is provided an oxidation catalyst disposed in the reaction duct. Furthermore, a hydrolysis catalyst may be disposed in the housing upstream of the reduction catalyst, relative to a flow of the exhaust gas. The hydrolysis catalyst is disposed in the conversion and mixing duct, in the deflector, or in the reaction duct.
In accordance with yet an added feature of the invention, the conversion and mixing duct is disposed centrally within the reaction duct.
In accordance with yet an additional feature of the invention, the reduction catalyst (e.g. an SCR catalyst) is a catalytic converter formed with individual modules, and the conversion and mixing duct has a cross-section selected from the group consisting of rectangular, round, and rectangular with rounded corners. The individual modules have a parallelepiped and/or circular-segment cross section.
In accordance with yet another feature of the invention, the housing is formed with a prechamber disposed between the exhaust gas inlet in the housing and the conversion and mixing duct.
In accordance with yet a further feature of the invention, the conversion and mixing duct has an intake formed with at least two inlet orifices for the exhaust gas.
In accordance with yet a supplemental feature of the invention, the deflector(s) is provided with a catalytically active surface.
In accordance with yet another feature of the invention, there is provided a silencer in the reaction duct.
With the principle of this invention, the reaction duct can be arranged parallel and adjacent to the conversion and mixing duct (xe2x80x9cparallel typexe2x80x9d). However, in a particularly preferred embodiment, the conversion and mixing duct is arranged in the middle area, specifically centrally in the reaction duct (xe2x80x9ccentral typexe2x80x9d). Accordingly, a catalytic reactor configuration results in this invention with individual catalyst modules that make up the system. In-the middle areaxe2x80x94preferably centrallyxe2x80x94the inner duct (the conversion and mixing duct) is formed through which the exhaust gas flows. The preferred catalytic reactor has catalyst modules arranged in p rows and in q columns and the middle areaxe2x80x94preferably centrallyxe2x80x94leaves free (mxc3x97n) catalyst modules to form the conversion and mixing duct. The variables m, n, p, and q are integers and m less than p and n less than q. The variables m and p pertain to the same direction (e.g. in the Cartesian system) and the variables n and q pertain to the same direction (orthogonal to m, p). According to another advantageous construction, a catalyst of circular ring shape is provided which is composed of a number of circular-segment catalyst modules whose inner region is provided to form a conversion and mixing duct of round cross section and which can be inserted into a housing of round external shape. In one specific configuration, the catalytic reactor body is rounded off externally, where parallelepiped and circular-segment modules are used.
By means of the device according to the invention, a particularly compact structure can be achieved, since only two mutually parallel ducts or only two ducts arranged one inside the other are required. The volume of the structure can therefore be kept small in relation to the catalyst volume. Owing to the highly simple, virtually symmetrical flow distribution in preferred embodiments, the construction principle can be retained for different sizes. There is therefore the potential for retaining the construction principle (xe2x80x9cscale-upxe2x80x9d), which is highly desirable for obvious cost reasons.
There is also provided, in accordance with the invention, a combination of the device for cleaning exhaust gas and a catalytic converter. The device has a combined conversion and mixing duct defined therein in which exhaust gas is mixed with a reducing agent injected into the conversion and mixing duct, a reaction duct extending substantially parallel to the conversion and mixing duct, and a deflector disposed between the conversion and mixing duct and the reaction duct, the deflector deflecting the exhaust gas from a flow direction in the conversion and mixing duct into an opposite flow direction in the reaction duct. The catalytic converter comprises a plurality of catalyst modules defining an inner duct in a middle area thereof, the exhaust gas flowing through the inner duct.
In accordance with another feature of the invention, the inner duct is disposed centrally within the plurality of catalyst modules.
In accordance with an additional feature of the invention, the modules are rectangular catalyst modules disposed in a checkered pattern along p rows and q columns, and wherein, in the middle area, mxc3x97n modules are omitted to form the inner duct, and wherein m, n, p, and q are integers, m less than p, and n less than q.
In accordance with again another feature of the invention, the modules are circular-segment catalyst modules defining a circular ring shape of the catalytic converter, wherein the inner duct has a round cross section, and wherein the catalytic converter is insertable into a housing having a round outer contour.
In accordance with a concomitant feature of the invention, the modules are a plurality of rectangular catalyst modules and a plurality of circular-segment catalyst modules defining a substantially rectangular catalytic converter with rounded corners.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a process for the catalytic cleaning of the exhaust gas from a combustion plant, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.